This invention relates to articles comprising graphite and more particularly to high tensile modulus compositions comprising highly oriented graphite and methods for the production of such compositions.
Carbon structures are widely used for applications where high temperatures will be encountered and where heat dissipation is important such as, for example, in high energy brake pads and in consumer electronic devices as electronic heat sinks. While a good balance of mechanical properties continues to be important in such demanding applications, high thermal conductivity and good dimensional stability have become particularly important considerations. The thermal conductivity and dimensional stability of solid carbon depends largely on its structure. Characteristically, these properties improve as the crystallinity and density of the carbon increases. Solid amorphous carbon may typically have a density near 1.2 g/cc and a thermal conductivity as low as 100 w/m-.degree.K., while single crystal graphite has a density of about 2.26 g/cc, a thermal conductivity near 1800 w/m-.degree.K.--considerably greater than the conductivity of copper--and, unlike metals, a negative coefficient of thermal expansion. These characteristics are highly desired by users of carbon articles, and the art has expended considerable effort seeking methods for producing carbon structures with such high densities reproducibly and with good control.
Highly ordered pyrolytic graphites having densities near 2.2 g/cc and good thermal conductivity have been produced by vapor deposition of carbon. Highly oriented pyrolytic graphite (HOPG) may have a thermal conductivity on the order of 800 w/m-.degree.K. However, the HOPG materials are extremely fragile, too brittle even for measurement of mechanical properties such as tensile strength, and are extremely costly to produce. The process is extremely costly, and is capable only of producing very small, extremely fragile, wafer-like articles on the order of about one-half to one inch square. HOPG materials are thus severely limited in their application and have not found wide acceptance.
The bulk graphites widely used commercially for fabricating articles such as crucibles, electrodes and the like are largely amorphous and relatively low in density, and lack the high thermal conductivity of crystal graphite. To the extent particular bulk graphites may be crystalline, the crystalline component will comprise large, randomly-oriented graphitic crystallites, generally greater in size than about 30 to 50 microns, embedded in a substantially amorphous carbon phase. These lower-density bulk graphite articles will generally exhibit only a fraction of the bulk thermal conductivity that characterizes highly organized crystalline graphite. The degree of crystallinity in bulk graphite structures may be altered by optinfizing a number of process factors including annealing and by the nature of the pitch employed. Mesophase or liquid crystal pitch may be readily transformed thermally into a more crystalline bulk graphite; however, bulk mesophase pitch is generally not oriented and, when processed in bulk into crystalline graphite, the crystallites also lack orientation. Although the density of these bulk graphite articles thus may be higher than for other forms of bulk carbon, the bulk thermal conductivity is considerably below that of crystal graphite.
Bulk graphites also lack the mechanical strength needed for more demanding thermal applications. Adding carbon or graphite fiber reinforcement directly to bulk pitch prior to thermal processing may afford modest improvement in mechanical properties. As with most composite materials, control of fiber reinforcement configuration through use of carbon fiber fabric or other structured preforms may permit further improvement in properties. Infiltrating a carbon fiber preform with pitch or vapor-deposited pyrolytic carbon to serve as binder and matrix, then carbonizing and graphitizing, will provide composites with improved the mechanical properties. However, even with use of pressure consolidation, the articles will generally have densities below about 1.5 g/cc and correspondingly low thermal conductivities, generally below about 500 w/m-.degree.K. In addition, infiltrating a preform with pitch or with a vapor-deposited carbon is difficult, time-consuming and expensive.
One widely-used method used for producing reinforced carbon articles has been to coat sheets of graphite cloth with a suitable binder, stack the sheets and heat the structure to carbonize the binder. In U.S. Pat. No. 4,178,413 a process is disclosed whereby a woven carbon fiber structure is formed, for example, from carbonized cloth of rayon or polyacrylonitrile (PAN), infiltrated with a vapor-deposited pyrolyric carbon to bond the substrate fiber, then impregnated with a carbonizable filler, cured under pressure and finally carbonized. Five to ten cycles of impregnating and carbonizing are necessary to produce a carbon article having a density suitable for carbon brake use, disclosed therein as in the range of 1.5 to 1.85 g/cc. Such processes are extremely difficult to carry out without introducing variation in density,, void formation and cracking.
An alternative to pitch infiltration processes, disclosed in U.S. Pat. No. 4,849,200, employs a preform constructed from an intimate combination of pitch fiber and a pitch-based carbon fiber reinforcement. When placed under an applied pressure of at least 10 kg/cm.sup.2 and fully carbonized thermally, the pitch fiber component apparently melts and flows to supply the matrix component of the composite cementing the reinforcing fiber. The volume fraction of the fiber reinforcement in the resulting composite will generally be less than about 70 volume %, and the bulk density of the composites is seen to be generally less than about 1.7 g/cc.
Binderless processes involving thermal processing of a carbonized pitch fiber bundle are also known. These processes are ordinarily carried out using extreme pressures, externally applied, to compact the structure and force the carbonized pitch to flow and cement the fiber bundle. For example, U.S. Pat. No. 4,032,607 discloses forming staple lengths of fiber by spinning a carbonaceous pitch, preferably by blow spinning, and depositing the fiber on a screen to form a web. The web is then heated in air to oxidize the fiber surfaces to an oxygen level of 1 to about 6 wt. %, which generally is sufficient to stabilize the fiber mat or felt without completely thermosetting the fiber and rendering the fiber infusible. Further heating in an inert atmosphere under pressure causes unoxidized pitch to flow and exude through defects in the fiber, providing a pitch matrix to bind the fiber. Carbonizing the structure provides a low-density carbon composite with a high degree of porosity.
As disclosed in U.S. Pat. No. 4,350,672, a preform made from acrylic fiber oxidized to a level of oxygen sufficient to render the fiber non-melting is first consolidated by applying heat and pressure and then carbonized and graphitized by heating in an inert gas atmosphere, providing a carbon body said to have a fibular microstructure and to be porous, the level of porosity ranging from 2% for very high consolidation pressures to greater than 70% when lower consolidation pressures are employed. The consolidation step is characterized as causing individual fibers to bond together, with heat distortion flow increasing the contact area of the fibers and promoting bonding between contiguous fibers. These structures may be more appropriately described as porous, sintered, fibrous bodies.
In U.S. Pat. No. 4,777,093, there is disclosed a process wherein pre-oxidized PAN fiber having an oxygen content in the range of 9 to 14 wt. % is first subjected to a series of forming operations to produce lengths of densifted tow having fiber density of up to about 75 to 80%, then infused with water or other suitable plasticiser to swell the fiber and leach low polymer from the fiber interior. The tow structure is then encapsulated with low temperature metal alloy and subjected to hot isostatic pressing at pressures as great as 15,000 psi. After first melting and removing the metal alloy, the resulting carbon body may then be graphitized by heating under inert atmosphere at temperatures as great as 2500.degree.-3200.degree. C. to have a density as great as 2.1 g/cc and a thermal conductivity in the range of 350-400 w/m.degree.K. Although the conductivity is generally higher than for graphitized PAN fiber, it is inadequate for most thermal applications. In addition, the rigidity of the graphitized structure is undesirably low, with modulus values for the graphitized body generally below 50.times.10.sup.6 psi.
It is apparent that the processes heretofore available in the art for producing high density carbon articles are generally unsatisfactory. Most are very expensive to practice, requiring specialized equipment capable of achieving high pressures and temperatures, and may require inordinately long times, often on the order of months to complete, further adding to cost. Highly ordered graphites are generally brittle, while reinforced structures lack the necessary thermal properties. Generally, the better reinforced carbon articles known in the art have thermal conductivities less than about 300 w/m-.degree.K., while most bulk graphites exhibit lower thermal conductivity, even as low as 50 w/m-.degree.K. Such carbon articles also generally lack the highly desirable negative coefficient of thermal expansion characteristic of crystal graphite, and many also are deficient in mechanical properties, particularly tensile strength and rigidity.
The demand for carbon articles that combine high thermal conductivity, a desirable balance of mechanical properties including good tensile properties and high modulus, greater than 70.times.10.sup.6 psi, and a negative coefficient of thermal expansion continues to grow. A high degree of dimensional stability, rigidity at high temperatures and excellent thermal conductivity are, in combination, increasingly important design requirements, and are particularly desired for applications where weight reduction is important, for example in consumer goods including electronic devices. Carbon structures exhibiting these highly desired properties and preferably with a high degree of anisotropy, that is, directed as desired along an axis of the structure, and a method for making such structures would be particularly useful and desirable for constructing heat sinks for electronic devices and in the design of brake materials for very high friction loads.